Mitosis in filamentous fungi: how we got where we are

Nup2 performs diverse interphase functions in Aspergillus nidulans

The nuclear pore complex (NPC) protein Nup2 plays interphase nuclear transport roles and in Aspergillus nidulans also functions to bridge NPCs at mitotic chromatin for their faithful coinheritance to daughter G1 nuclei. In this study, we further investigate the interphase functions of Nup2 in A. nidulans. Although Nup2 is not required for nuclear import of all nuclear proteins after mitosis, it is required for normal G1 nuclear accumulation of the NPC nuclear basket–associated components Mad2 and Mlp1 as well as the THO complex protein Tho2. Targeting of Mlp1 to nuclei partially rescues the interphase delay seen in nup2 mutants indicating that some of the interphase defects in Nup2-deleted cells are due to Mlp1 mislocalization. Among the inner nuclear membrane proteins, Nup2 affects the localization of Ima1, orthologues of which are involved in nuclear movement. Interestingly, nup2 mutant G1 nuclei also exhibit an abnormally long period of extensive to-and-fro movement immediately after mitosis in a manner dependent on the microtubule cytoskeleton. This indicates that Nup2 is required to limit the transient postmitotic nuclear migration typical of many filamentous fungi. The findings reveal that Nup2 is a multifunctional protein that performs diverse functions during both interphase and mitosis in A. nidulans.

Fungal Cell Cycle: A Unicellular versus Multicellular Comparison

All cells must accurately replicate DNA and partition it to daughter cells. The basic cell cycle machinery is highly conserved among eukaryotes. Most of the mechanisms that control the cell cycle were worked out in fungal cells, taking advantage of their powerful genetics and rapid duplication times. Here we describe the cell cycles of the unicellular budding yeast Saccharomyces cerevisiae and the multicellular filamentous fungus Aspergillus nidulans. We compare and contrast morphological landmarks of G1, S, G2, and M phases, molecular mechanisms that drive cell cycle progression, and checkpoints in these model unicellular and multicellular fungal systems.

Dynamics of the establishment of multinucleate compartments in Fusarium oxysporum

Nuclear dynamics can vary widely between fungal species and between stages of development of fungal colonies. Here we compared nuclear dynamics and mitotic patterns between germlings and mature hyphae in Fusarium oxysporum. Using fluorescently labeled nuclei and live-cell imaging, we show that F. oxysporum is subject to a developmental transition from a uninucleate to a multinucleate state after completion of colony initiation. We observed a special type of hypha that exhibits a higher growth rate, possibly acting as a nutrient scout. The higher growth rate is associated with a higher nuclear count and mitotic waves involving 2 to 6 nuclei in the apical compartment. Further, we found that dormant nuclei of intercalary compartments can reenter the mitotic cycle, resulting in multinucleate compartments with up to 18 nuclei in a single compartment.

Spore development and nuclear inheritance in arbuscular mycorrhizal fungi

BACKGROUND

A conventional tenet of classical genetics is that progeny inherit half their genome from each parent in sexual reproduction instead of the complete genome transferred to each daughter during asexual reproduction. The transmission of hereditary characteristics from parents to their offspring is therefore predictable, although several exceptions are known. Heredity in microorganisms, however, can be very complex, and even unknown as is the case for coenocytic organisms such as Arbuscular Mycorrhizal Fungi (AMF). This group of fungi are plant-root symbionts, ubiquitous in most ecosystems, which reproduce asexually via multinucleate spores for which sexuality has not yet been observed.

RESULTS

We examined the number of nuclei per spore of four AMF taxa using high Z-resolution live confocal microscopy and found that the number of nuclei was correlated with spore diameter. We show that AMF have the ability, through the establishment of new symbioses, to pass hundreds of nuclei to subsequent generations of multinucleated spores. More importantly, we observed surprising heterogeneity in the number of nuclei among sister spores and show that massive nuclear migration and mitosis are the mechanisms by which AMF spores are formed. We followed spore development of Glomus irregulare from hyphal swelling to spore maturity and found that the spores reached mature size within 30 to 60 days, and that the number of nuclei per spores increased over time.

CONCLUSIONS

We conclude that the spores used for dispersal of AMF contain nuclei with two origins, those that migrate into the spore and those that arise by mitosis in the spore. Therefore, these spores do not represent a stage in the life cycle with a single nucleus, raising the possibility that AMF, unlike all other known eukaryotic organisms, lack the genetic bottleneck of a single-nucleus stage.

Nuclear dynamics, mitosis, and the cytoskeleton during the early stages of colony initiation in Neurospora crassa

Neurospora crassa macroconidia form germ tubes that are involved in colony establishment and conidial anastomosis tubes (CATs) that fuse to form interconnected networks of conidial germlings. Nuclear and cytoskeletal behaviors were analyzed in macroconidia, germ tubes, and CATs in strains that expressed fluorescently labeled proteins. Heterokaryons formed by CAT fusion provided a rapid method for the imaging of multiple labeled fusion proteins and minimized the potential risk of overexpression artifacts. Mitosis occurred more slowly in nongerminated macroconidia (1.0 to 1.5 h) than in germ tubes (16 to 20 min). The nucleoporin SON-1 was not released from the nuclear envelope during mitosis, which suggests that N. crassa exhibits a form of "closed mitosis." During CAT homing, nuclei did not enter CATs, and mitosis was arrested. Benomyl treatment showed that CAT induction, homing, fusion, as well as nuclear migration through fused CATs do not require microtubules or mitosis. Three ropy mutants (ro-1, ro-3, and ro-11) defective in the dynein/dynactin microtubule motor were impaired in nuclear positioning, but nuclei still migrated through fused CATs. Latrunculin B treatment, imaging of F-actin in living cells using Lifeact-red fluorescent protein (RFP), and analysis of mutants defective in the Arp2/3 complex demonstrated that actin plays important roles in CAT fusion.

Chromosome visualisation in filamentous fungi

Many attempts have been made to study the chromosomes of fungi, but a major problem is that fungal nuclei are so small. Fungal chromosomes are at the lowest resolution of light microscopy; thus few attempts to visualise fungal chromosomes have been successful. Fungi examined have been mainly Ascomycotina. The number of chromosomes per nucleus, estimated by conventional light visualisation and stained with standard dyes like Giemsa or aceto-orceine, usually does not exceed ten. A method developed in the late 1980s called 'germ tube burst' enables the discharge of condensed chromosomes from the hyphal cell and their spread on the surface of a slide. This more accurate method, usually gives better resolution of chromosomes. It was used with conventional light microscopy dyes as well as in fluorescent microscopy or for in situ hybridisation. A breakthrough has been made in fungal genetics by using pulse field gel electrophoresis (PFGE). Separation of the chromosomes on the gel enables the determination of their number and estimation of genome size. It also reveals chromosome length polymorphism and the presence of supernumerary chromosomes, which are usually too small to visualise in nuclei. A combination of two methods, cytological analysis by light microscopy and PFGE, should give a tool allowing the complex analysis of fungal genomes.

The Kip3-like kinesin KipB moves along microtubules and determines spindle position during synchronized mitoses in Aspergillus nidulans hyphae

Kinesins are motor proteins which are classified into 11 different families. We identified 11 kinesin-like proteins in the genome of the filamentous fungus Aspergillus nidulans. Relatedness analyses based on the motor domains grouped them into nine families. In this paper, we characterize KipB as a member of the Kip3 family of microtubule depolymerases. The closest homologues of KipB are Saccharomyces cerevisiae Kip3 and Schizosaccharomyces pombe Klp5 and Klp6, but sequence similarities outside the motor domain are very low. A disruption of kipB demonstrated that it is not essential for vegetative growth. kipB mutant strains were resistant to high concentrations of the microtubule-destabilizing drug benomyl, suggesting that KipB destabilizes microtubules. kipB mutations caused a failure of spindle positioning in the cell, a delay in mitotic progression, an increased number of bent mitotic spindles, and a decrease in the depolymerization of cytoplasmic microtubules during interphase and mitosis. Meiosis and ascospore formation were not affected. Disruption of the kipB gene was synthetically lethal in combination with the temperature-sensitive mitotic kinesin motor mutation bimC4, suggesting an important but redundant role of KipB in mitosis. KipB localized to cytoplasmic, astral, and mitotic microtubules in a discontinuous pattern, and spots of green fluorescent protein moved along microtubules toward the plus ends.

The csnD/csnE signalosome genes are involved in the Aspergillus nidulans DNA damage response

The signalosome (CSN) is a conserved multiprotein complex involved in regulation of eukaryotic development and is also required to activate ribonucleotide reductase for DNA synthesis. In Aspergillus nidulans, csnD/csnE are key regulators of sexual development. Here, we investigated whether the csnD/csnE genes are involved in the DNA damage response in this fungus. The growth of the csnD/csnE deletion mutants was reduced by subinhibitory concentrations of hydroxyurea, camptothecin, 4-nitroquinoline oxide, and methyl methanesulfonate. A. nidulans increases csnD/csnE mRNA levels when it is challenged by different DNA-damaging agents. There is no significant transcriptional induction of the csnE promoter fused with lacZ gene in the presence of DNA-damaging agents, suggesting that increased mRNA accumulation is due to increased mRNA stability. Septation was not inhibited in the csnD/csnE deletion mutants while DeltauvsB DeltacsnE presented an increase in septation upon DNA damage caused by methyl methanesulfonate, suggesting that uvsB(ATR) and csnE genetically interact during checkpoint-dependent inhibition of septum formation. The double DeltacsnD/DeltacsnE DeltanpkA mutants were more sensitive to DNA-damaging agents than were the respective single mutants. Our results suggest that csnD/csnE genes are involved in the DNA damage response and that NpkA and UvsB(ATR) genetically interact with the signalosome.

Dynamic association of topoisomerase II to the mitotic chromosomes in live cells of Aspergillus nidulans

DNA topoisomerase II (Topo II) is an essential enzyme that catalyzes topological changes of DNA and consists of a major member of mitotic chromosomes. To investigate the dynamic localization of Topo II in nuclei, we engineered the strain of Aspergillus nidulans expressing Topo II fused with green fluorescent protein (GFP). Time-lapse microscopy revealed that the distribution of Topo II-GFP in nuclei varied depending on the cell cycle. In interphase, Topo II-GFP distributed evenly in the nucleoplasm and at the onset of G2 phase became concentrated into nucleolus. During mitosis, Topo II-GFP accumulated on chromosomes, when the chromosomes condensed. In the early mitosis, the Topo II also showed a single or two brighter spots among the fluorescence of clumped chromosomes. The spots once divided into several spots and then concentrated again into a spot per nucleus in the dividing nuclei of anaphase. Along with the subsequent decondensation of chromosomes, Topo II diffused back into nucleoplasm.

The Aspergillus nidulans npkA gene encodes a Cdc2-related kinase that genetically interacts with the UvsBATR kinase

The DNA damage response is a protective mechanism that ensures the maintenance of genomic integrity. We have used Aspergillus nidulans as a model system to characterize the DNA damage response caused by the antitopoisomerase I drug, camptothecin. We report the molecular characterization of a p34Cdc2-related gene, npkA, from A. nidulans. The npkA gene is transcriptionally induced by camptothecin and other DNA-damaging agents, and its induction in the presence of camptothecin is dependent on the uvsBATR gene. There were no growth defects, changes in developmental patterns, increased sensitivity to DNA-damaging agents, or effects on septation or growth rate in the A. nidulans npkA deletion strain. However, the DeltanpkA mutation can partially suppress HU sensitivity caused by the DeltauvsBATR and uvsD153ATRIP checkpoint mutations. We demonstrated that the A. nidulans uvsBATR gene is involved in DNA replication and the intra-S-phase checkpoints and that the DeltanpkA mutation can suppress its intra-S-phase checkpoint deficiency. There is a defect in both the intra-S-phase and DNA replication checkpoints due to the npkA inactivation when DNA replication is slowed at 6 mm HU. Our results suggest that the npkA gene plays a role in cell cycle progression during S-phase as well as in a DNA damage signal transduction pathway in A. nidulans.

Dual-Color imaging of nuclear division and mitotic spindle elongation in live cells of Aspergillus nidulans

We have developed a dual-color imaging system based on cyan fluorescent protein-labeled histone H2A and green fluorescent protein-labeled alpha tubulin to visualize DNA and spindles simultaneously in the same living cell of Aspergillus nidulans. This system allows new details of mitosis and nuclear movement to be revealed.

GFP as a tool to analyze the organization, dynamics and function of nuclei and microtubules in Neurospora crassa

We report the construction of a versatile GFP expression plasmid and demonstrate its utility in Neurospora crassa. To visualize nuclei and microtubules, we generated carboxy-terminal fusions of sgfp to Neurospora histone H1 (hH1) and beta-tubulin (Bml). Strong expression of GFP fusion proteins was achieved with the inducible Neurospora ccg-1 promoter. Nuclear and microtubule organization and dynamics were observed in live vegetative hyphae, developing asci, and ascospores by conventional and confocal laser scanning fluorescence microscopy. Observations of GFP fusion proteins in live cells largely confirmed previous results obtained by examination of fixed cells with various microscopic techniques. H1-GFP revealed dynamic nuclear shapes. Microtubules were mostly aligned parallel to the growth axis in apical compartments but more randomly arranged in sub-apical compartments. Time-lapse imaging of beta-tubulin-GFP in germinating macroconidia revealed polymerization and depolymerization of microtubules. In heterozygous crosses, H1-GFP and beta-tubulin-GFP expression was silenced, presumably by meiotic silencing. H1-GFP was translated in the vicinity of hH1+-sgfp+ nuclei in the common cytoplasm of giant Banana ascospores, but it diffused into all nuclei, another illustration of the utility of GFP fusion proteins.

Polarity in filamentous fungi: moving beyond the yeast paradigm

Filamentous fungi grow by the polar extension of hyphae. This polar growth requires the specification of sites of germ tube or branch emergence, followed by the recruitment of the morphogenetic machinery to those sites for localized cell wall deposition. Researchers attempting to understand hyphal morphogenesis have relied upon the powerful paradigm of bud emergence in the yeast Saccharomyces cerevisiae. The yeast paradigm has provided a useful framework, however several features of hyphal morphogenesis, such as the ability to maintain multiple axes of polarity and an extremely rapid extension rate, cannot be explained by simple extrapolation from yeast models. We discuss recent polarity research from filamentous fungi focusing on the position of germ tube emergence, the relaying of positional information via RhoGTPase modules, and the recruitment of morphogenetic machinery components including cytoskeleton, polarisome and ARP2/3 complexes, and the vesicle trafficking system.

Aspergillus nidulans as a model system to characterize the DNA damage response in eukaryotes

Interest in DNA repair in Aspergillus nidulans had mainly grown out of studies of three different biological processes, namely mitotic recombination, inducible responses to detrimental environmental changes, and genetic control of the cell cycle. Ron Morris started the investigation of the genetic control of the cell cycle by screening hundreds of cell cycle temperature sensitive Aspergillus mutants. The sequencing and innovative analysis of these genes revealed not only several components of the cell cycle machinery that are directly involved in checkpoint response, but also components required for DNA replication and DNA damage response machinery. Here, we will provide an overview about currently known aspects of the DNA damage response in A. nidulans. Emphasis is put on analyzed mutants that are available and review epistatic relationships and other interactions among them. Furthermore, a comprehensive list of A. nidulans genes involved in different processes of the DNA damage response, as identified by homology of genome sequences with well-characterized human and yeast DNA repair genes, is shown.

Different roles of the Mre11 complex in the DNA damage response in Aspergillus nidulans

The Mre11-Rad50-Nbs1 protein complex has emerged as a central player in the cellular DNA damage response. Mutations in scaANBS1, which encodes the apparent homologue of human Nbs1 in Aspergillus nidulans, inhibit growth in the presence of the anti-topoisomerase I drug camptothecin. We have used the scaANBS1 cDNA as a bait in a yeast two-hybrid screening and report the identification of the A. nidulans Mre11 homologue (mreA). The inactivated mreA strain was more sensitive to several DNA damaging and oxidative stress agents. Septation in A. nidulans is dependent not only on the uvsBATR gene, but also on the mre11 complex. scaANBS1 and mreA genes are both involved in the DNA replication checkpoint whereas mreA is specifically involved in the intra-S-phase checkpoint. ScaANBS1 also participates in G2-M checkpoint control upon DNA damage caused by MMS. In addition, the scaANBS1 gene is also important for ascospore viability, whereas mreA is required for successful meiosis in A. nidulans. Consistent with this view, the Mre11 complex and the uvsCRAD51 gene are highly expressed at the mRNA level during the sexual development.

The art and design of genetic screens: filamentous fungi

In the 1940s, screens for metabolic mutants of the filamentous fungus Neurospora crassa established the fundamental, one-to-one relationship between a gene and a specific protein, and also established fungi as important genetic organisms. Today, a wide range of filamentous species, which represents a billion years of evolutionary divergence, is used for experimental studies. The developmental complexity of these fungi sets them apart from unicellular yeasts, and allows the development of new screens that enable us to address biological questions that are relevant to all eukaryotes.

Mitosis and motor proteins in the filamentous ascomycete, Nectria haematococca, and some related fungi

Among filamentous fungi, mitosis has been studied in-depth in just a few species. The mitotic apparatuses in the ascomycetous Fusarium spp. are the most clearly and readily visualized in vivo within this group; fluorescent labeling is unnecessary. This superior cytological tractability has enabled detailed studies and revealing experiments that have led the way toward a more complete understanding of fungal mitosis. Some of the most important discoveries include the role of half-spindles in development of the bipolar spindle, the existence of true kinetochores in fungi, the unorthodox chromosome configurations and movements comprising metaphase and anaphase A, the attachment of astral microtubules to the plasmalemma, the role of the astral pulling force in elongating the spindle, an inwardly directed force within the spindle, and microtubule cross-bridging in both spindle and asters. Recent research has focused on the roles of microtubuleassociated motor proteins in Fusarium solani f. sp. pisi (anamorph of Nectria haematococca). Cytoplasmic dynein was shown to be involved in the development and/or maintenance of mitotic asters and necessary for motility and functionality of the interphase spindle pole body. The inwardly directed force within the anaphase spindle was shown to be produced by a kinesin-related protein, NhKRP1. Because of its superior cytological tractability, the considerable and unique knowledge we have of many aspects of its mitosis, and its genetic tractability, Fusarium solani f. sp. pisi is a good choice for further investigations of mitosis in filamentous fungi.